Linking nutrient sensing, mitochondrial function, and PRR immune cell signaling in liver disease
Authors: Claudia Kemper, Michael N. Sack
Journal: Trends in Immunology (2022)

Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic hepatic steatohepatitis (NASH) are on the rise, and therapeutics to successfully target these detrimental conditions are very limited.Nutrient-induced hepatic lipid overload across cells in the mammalian liver contributes to mitochondrial dysfunction, producing damage-associated molecular patterns (DAMPs) that initiate hepatosteatosis.Pattern recognition receptors (PRRs) integrate mitochondrial perturbations into proinflammatory cellular behavior in mammalian liver parenchymal cells and immune cells, supporting NAFLD pathogenesis and progression to NASH.Intracellular components of the canonical PRR system, complement (the complosome), operate across a range of cells in humans and mice, and may contribute to NAFLD/NASH pathogenesis.Pharmacologically targeting nutrient–mitochondria–PRR pathways, including intracellularly active complement, might lead to new putative NAFLD/NASH treatments. The mechanistic understanding of the recently identified functional interplay between nutrient sensing, mitochondrial activity, classic PRR, and intracellular complement activation across cell populations may help to uncover novel therapeutic intervention targets to treat nonalcoholic fatty liver disease. Caloric overconsumption in vertebrates promotes adipose and liver fat accumulation while perturbing the gut microbiome. This triad triggers pattern recognition receptor (PRR)-mediated immune cell signaling and sterile inflammation. Moreover, immune system activation perpetuates metabolic consequences, including the progression of nonalcoholic fatty liver disease (NAFLD) to nonalcoholic hepatic steatohepatitis (NASH). Recent findings show that sensing of nutrient overabundance disrupts the activity and homeostasis of the central cellular energy-generating organelle, the mitochondrion. In parallel, whether caloric excess-initiated PRR signaling and mitochondrial perturbations are coordinated to amplify this inflammatory process in NASH progression remains in question. We hypothesize that altered mitochondrial function, classic PRR signaling, and complement activation in response to nutrient overload together play an integrated role across the immune cell landscape, leading to liver inflammation and NASH progression. Caloric overconsumption in vertebrates promotes adipose and liver fat accumulation while perturbing the gut microbiome. This triad triggers pattern recognition receptor (PRR)-mediated immune cell signaling and sterile inflammation. Moreover, immune system activation perpetuates metabolic consequences, including the progression of nonalcoholic fatty liver disease (NAFLD) to nonalcoholic hepatic steatohepatitis (NASH). Recent findings show that sensing of nutrient overabundance disrupts the activity and homeostasis of the central cellular energy-generating organelle, the mitochondrion. In parallel, whether caloric excess-initiated PRR signaling and mitochondrial perturbations are coordinated to amplify this inflammatory process in NASH progression remains in question. We hypothesize that altered mitochondrial function, classic PRR signaling, and complement activation in response to nutrient overload together play an integrated role across the immune cell landscape, leading to liver inflammation and NASH progression. Sustained caloric excess leads to metabolic syndrome, an umbrella term that incorporates at least four pathophysiological conditions including obesity, insulin resistance, hypercholesterolemia, and hypertension. Metabolic syndrome, in turn, increases stroke, cardiovascular disease (CVD), and type 2 diabetes mellitus (T2DM) risk [1.O'Neill S. O'Driscoll L. Metabolic syndrome: a closer look at the growing epidemic and its associated pathologies.Obes. Rev. 2015; 16: 1-12Crossref PubMed Scopus (1017) Google Scholar]. NAFLD (see Glossary) is a further manifestation of metabolic syndrome that encompasses initial excessive fat infiltration into the liver (steatosis), which can progress to reversible NASH and the potential to progress towards non-reversible liver cirrhosis, increasing the risk of hepatocellular carcinoma (HCC) development [2.Anstee Q.M. et al.From NASH to HCC: current concepts and future challenges.Nat. Rev. Gastroenterol. Hepatol. 2019; 16: 411-428Crossref PubMed Scopus (710) Google Scholar]. The full spectrum of NAFLD constitutes the most common chronic liver disease and its prevalence is increasing in parallel with the global rise in obesity and metabolic syndrome [3.Estes C. et al.Modeling the epidemic of nonalcoholic fatty liver disease demonstrates an exponential increase in burden of disease.Hepatology. 2018; 67: 123-133Crossref PubMed Scopus (1180) Google Scholar]. Although genetic susceptibility is evident [4.Baker 2nd, P.R. Friedman J.E. Mitochondrial role in the neonatal predisposition to developing nonalcoholic fatty liver disease.J. Clin. Invest. 2018; 128: 3692-3703Crossref PubMed Scopus (37) Google Scholar], the increasing prevalence implicates environmental triggers where excess caloric load is a major driver. Under physiological conditions, hepatocyte lipid deposition functions as a dynamic lipid storage site during eating and fasting, and a concomitant regulated machinery controls lipid droplet (LD) synthesis and lipolysis [5.Kramer D.A. et al.Fasting and refeeding induces changes in the mouse hepatic lipid droplet proteome.J. Proteome. 2018; 181: 213-224Crossref PubMed Scopus (29) Google Scholar]. Sustained nutritional overload associated with NAFLD overwhelms these regulatory programs, leading to steatosis [6.Gluchowski N.L. et al.Lipid droplets and liver disease: from basic biology to clinical implications.Nat. Rev. Gastroenterol. Hepatol. 2017; 14: 343-355Crossref PubMed Scopus (334) Google Scholar]. Subsequently, hepatocyte functional perturbations and cell death promote the development of systemic and local (chronic) inflammation; in parallel with induced liver fibrosis, changes in liver-resident and infiltrative immune cell activity contribute to the progression to NASH (Box 1) – a precursor to exacerbated hepatic and systemic consequences [7.Schuster S. et al.Triggering and resolution of inflammation in NASH.Nat. Rev. Gastroenterol. Hepatol. 2018; 15: 349-364Crossref PubMed Scopus (452) Google Scholar]. For the latter, major contributors include an elevated state of activation of the vascular endothelium with concomitant atherosclerosis risk [8.Fracanzani A.L. et al.Carotid artery intima-media thickness in nonalcoholic fatty liver disease.Am. J. Med. 2008; 121: 72-78Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar] and an overconsumption-driven increase in intestinal permeability leading to gut microbe leakage [9.Henao-Mejia J. et al.Inflammasome-mediated dysbiosis regulates progression of NAFLD and obesity.Nature. 2012; 482: 179-185Crossref PubMed Scopus (1798) Google Scholar] – both of which provide additional sources of inflammatory signals.Box 1Liver cells and hepatic immune cellsThe mammalian liver comprises multiple cell types that play distinct functional roles depending on their localization around either the portal (intestinal) venous or hepatic arterial vascular supplies to the liver and subsequent drainage through the hepatic venous and hepatobiliary sinusoidal systems [85.Jenne C.N. Kubes P. Immune surveillance by the liver.Nat. Immunol. 2013; 14: 996-1006Crossref PubMed Scopus (705) Google Scholar]. The major hepatic parenchymal cell types linked to the pathophysiology of fatty liver include hepatocytes, the LSECs, and SCs (mediators of hepatic fibrosis) [7.Schuster S. et al.Triggering and resolution of inflammation in NASH.Nat. Rev. Gastroenterol. Hepatol. 2018; 15: 349-364Crossref PubMed Scopus (452) Google Scholar]. The liver also harbors a large range of resident immune cells including myeloid cells [e.g., macrophages called KCs and also dendritic cells (DCs)], innate T cells [γδ T cells, natural killer T cells (NKTs), and mucosa-associated invariant T cells (MAIT cells)], as well as adaptive αβ T cells and B cells [86.MacParland S.A. et al.Single cell RNA sequencing of human liver reveals distinct intrahepatic macrophage populations.Nat. Commun. 2018; 9: 4383Crossref PubMed Scopus (663) Google Scholar]. In the healthy liver, the general immune cell landscape is sustained mostly in an anti-inflammatory and tolerogenic state [7.Schuster S. et al.Triggering and resolution of inflammation in NASH.Nat. Rev. Gastroenterol. Hepatol. 2018; 15: 349-364Crossref PubMed Scopus (452) Google Scholar,85.Jenne C.N. Kubes P. Immune surveillance by the liver.Nat. Immunol. 2013; 14: 996-1006Crossref PubMed Scopus (705) Google Scholar]. During the progression to NASH, two principal events occur. First, liver cells and the resident hepatic immune cell repertoire are reshaped towards an activated state; second, a marked influx of almost all subtypes of immune cells into the inflamed liver can be seen, with myeloid cells dominating the recruited immune cells [87.Cai J. et al.The role of innate immune cells in nonalcoholic steatohepatitis.Hepatology. 2019; 70: 1026-1037Crossref PubMed Scopus (120) Google Scholar]. Together these events sustain the inflammatory environment that promotes liver injury. Although various immune cells affect different aspects of the pathogenesis of NAFLD, we currently know most about the contributions of myeloid cells (specifically of neutrophils, monocytes, and macrophages) and conventional T cells to liver disease [7.Schuster S. et al.Triggering and resolution of inflammation in NASH.Nat. Rev. Gastroenterol. Hepatol. 2018; 15: 349-364Crossref PubMed Scopus (452) Google Scholar,85.Jenne C.N. Kubes P. Immune surveillance by the liver.Nat. Immunol. 2013; 14: 996-1006Crossref PubMed Scopus (705) Google Scholar,87.Cai J. et al.The role of innate immune cells in nonalcoholic steatohepatitis.Hepatology. 2019; 70: 1026-1037Crossref PubMed Scopus (120) Google Scholar]. We therefore focus here on potential novel nutrient–PRR–mitochondria axes in these latter immune cell types. The mammalian liver comprises multiple cell types that play distinct functional roles depending on their localization around either the portal (intestinal) venous or hepatic arterial vascular supplies to the liver and subsequent drainage through the hepatic venous and hepatobiliary sinusoidal systems [85.Jenne C.N. Kubes P. Immune surveillance by the liver.Nat. Immunol. 2013; 14: 996-1006Crossref PubMed Scopus (705) Google Scholar]. The major hepatic parenchymal cell types linked to the pathophysiology of fatty liver include hepatocytes, the LSECs, and SCs (mediators of hepatic fibrosis) [7.Schuster S. et al.Triggering and resolution of inflammation in NASH.Nat. Rev. Gastroenterol. Hepatol. 2018; 15: 349-364Crossref PubMed Scopus (452) Google Scholar]. The liver also harbors a large range of resident immune cells including myeloid cells [e.g., macrophages called KCs and also dendritic cells (DCs)], innate T cells [γδ T cells, natural killer T cells (NKTs), and mucosa-associated invariant T cells (MAIT cells)], as well as adaptive αβ T cells and B cells [86.MacParland S.A. et al.Single cell RNA sequencing of human liver reveals distinct intrahepatic macrophage populations.Nat. Commun. 2018; 9: 4383Crossref PubMed Scopus (663) Google Scholar]. In the healthy liver, the general immune cell landscape is sustained mostly in an anti-inflammatory and tolerogenic state [7.Schuster S. et al.Triggering and resolution of inflammation in NASH.Nat. Rev. Gastroenterol. Hepatol. 2018; 15: 349-364Crossref PubMed Scopus (452) Google Scholar,85.Jenne C.N. Kubes P. Immune surveillance by the liver.Nat. Immunol. 2013; 14: 996-1006Crossref PubMed Scopus (705) Google Scholar]. During the progression to NASH, two principal events occur. First, liver cells and the resident hepatic immune cell repertoire are reshaped towards an activated state; second, a marked influx of almost all subtypes of immune cells into the inflamed liver can be seen, with myeloid cells dominating the recruited immune cells [87.Cai J. et al.The role of innate immune cells in nonalcoholic steatohepatitis.Hepatology. 2019; 70: 1026-1037Crossref PubMed Scopus (120) Google Scholar]. Together these events sustain the inflammatory environment that promotes liver injury. Although various immune cells affect different aspects of the pathogenesis of NAFLD, we currently know most about the contributions of myeloid cells (specifically of neutrophils, monocytes, and macrophages) and conventional T cells to liver disease [7.Schuster S. et al.Triggering and resolution of inflammation in NASH.Nat. Rev. Gastroenterol. Hepatol. 2018; 15: 349-364Crossref PubMed Scopus (452) Google Scholar,85.Jenne C.N. Kubes P. Immune surveillance by the liver.Nat. Immunol. 2013; 14: 996-1006Crossref PubMed Scopus (705) Google Scholar,87.Cai J. et al.The role of innate immune cells in nonalcoholic steatohepatitis.Hepatology. 2019; 70: 1026-1037Crossref PubMed Scopus (120) Google Scholar]. We therefore focus here on potential novel nutrient–PRR–mitochondria axes in these latter immune cell types. In the healthy mammalian liver, immune cells generally have a 'resting' or tolerogenic phenotype [10.Dou L. et al.Hepatic dendritic cells, the tolerogenic liver environment, and liver disease.Semin. Liver Dis. 2018; 38: 170-180Crossref PubMed Scopus (40) Google Scholar]. However, during the transition to NASH, danger signals and proinflammatory mediators released by injured and/or dying hepatocytes reshape the molecular signatures of resident immune and liver parenchymal cells via PRR engagement towards proinflammatory phenotypes [11.Zhou Z. et al.Neutrophil–hepatic stellate cell interactions promote fibrosis in experimental steatohepatitis.Cell. Mol. Gastroenterol. Hepatol. 2018; 5: 399-413Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar,12.Daemen S. et al.Dynamic shifts in the composition of resident and recruited macrophages influence tissue remodeling in NASH.Cell Rep. 2021; 34108626Abstract Full Text Full Text PDF PubMed Scopus (111) Google Scholar]. This, in combination with monocyte and neutrophil infiltration from the periphery, further augments liver dysfunction and fibrosis [13.Gadd V.L. et al.The portal inflammatory infiltrate and ductular reaction in human nonalcoholic fatty liver disease.Hepatology. 2014; 59: 1393-1405Crossref PubMed Scopus (296) Google Scholar] (Figure 1). Most studies assess single molecular pathways that contribute to liver disease in isolation. In this opinion article, we argue that there is a significant – and recently recognized – interplay between nutritional overload, PRR-mediated immune cell signaling, and the control of inflammatory cell programs that contribute to the progression of benign NAFLD to NASH. Further, we posit that, as a canonical nutrient-sensing intracellular organelle, the mitochondrion constitutes a central node that initiates these PRR-inflammatory programs. In addition, we suggest that the unexpected activities of intracellular complement may be an important constituent of metabolic and mitochondrial changes associated with NASH. Identifying novel mechanistic networks underlying NASH may uncover putative targets for urgently needed therapeutic interventions. We provide here a succinct overview of the known contributions of mitochondrial nutrient overload and subsequent engagement of classic PRRs such as Toll-like receptors (TLRs), inflammasomes, and cyclic GMP-AMP synthase–stimulator of interferon genes (cGAS–STING) pathways in NAFLD progression. We discuss recently discovered intracellular complement activities and how these may intersect with canonical hepatic PRR systems and mitochondrial perturbations during the progression to steatohepatitis. We also highlight potential new NASH/NAFLD pharmacological interventions targeting the proposed nutrient–PRR–mitochondria axes, and assess the limitations of this model as well as key outstanding questions. One principal function of the liver, and of hepatocytes, is the synthesis and storage of triglycerides (TGs) in LDs [6.Gluchowski N.L. et al.Lipid droplets and liver disease: from basic biology to clinical implications.Nat. Rev. Gastroenterol. Hepatol. 2017; 14: 343-355Crossref PubMed Scopus (334) Google Scholar]. It is therefore not surprising that this process is augmented in response to chronic nutritional overload. The accumulation of LDs themselves does not induce injury or inflammation because lipid encapsulation in the form of TGs neutralizes lipotoxic effects [6.Gluchowski N.L. et al.Lipid droplets and liver disease: from basic biology to clinical implications.Nat. Rev. Gastroenterol. Hepatol. 2017; 14: 343-355Crossref PubMed Scopus (334) Google Scholar]. However, associated perturbations of mitochondria, as nutrient-sensing intracellular organelles, can trigger retrograde signaling, often via intracellular PRR engagement, to both initiate and amplify inflammation [14.Garcia-Martinez I. et al.Hepatocyte mitochondrial DNA drives nonalcoholic steatohepatitis by activation of TLR9.J. Clin. Invest. 2016; 126: 859-864Crossref PubMed Scopus (327) Google Scholar]. Mitochondria are highly abundant in hepatocytes and function in part to supply energy for the liver, provide metabolic intermediates for cholesterol, steroid, bile acid, lipid, and glycogen synthesis, as well as in producing ketone bodies [15.Legaki A.I. et al.Hepatocyte mitochondrial dynamics and bioenergetics in obesity-related non-alcoholic fatty liver disease.Curr. Obes. Rep. 2022; 11: 126-143Crossref PubMed Scopus (17) Google Scholar]. NAFLD and NASH show progressive histological and biochemical perturbations in mitochondria, as evidenced by mitochondrial swelling, cristae disruption, and impaired mitochondrial oxidative phosphorylation [16.Wei Y. et al.Nonalcoholic fatty liver disease and mitochondrial dysfunction.World J. Gastroenterol. 2008; 14: 193-199Crossref PubMed Scopus (279) Google Scholar]. Of note, in obese human subjects, the loss of mitochondrial respiratory capacity and the increase in hepatic mitochondrial mass correlate with the transition from NAFLD to NASH [17.Koliaki C. et al.Adaptation of hepatic mitochondrial function in humans with non-alcoholic fatty liver is lost in steatohepatitis.Cell Metab. 2015; 21: 739-746Abstract Full Text Full Text PDF PubMed Scopus (604) Google Scholar]. The mechanism whereby perturbed mitochondrial fidelity promotes inflammation is driven in part by the evolutionary formation of these organelles as vestiges of their bacterial origins; indeed eukaryotic mitochondria retain numerous prokaryotic molecules, including hypomethylated mitochondrial DNA (mtDNA), formyl peptides, and cardiolipin [18.Calfee C.S. Matthay M.A. Clinical immunology: culprits with evolutionary ties.Nature. 2010; 464: 41-42Crossref PubMed Scopus (46) Google Scholar]. Leakage or extrusion of these components through the outer mitochondrial membrane leads to their recognition as damage-associated molecular patterns (DAMPs) [19.Sack M.N. Mitochondrial fidelity and metabolic agility control immune cell fate and function.J. Clin. Invest. 2018; 128: 3651-3661Crossref PubMed Scopus (21) Google Scholar]. Hence, disruption of mitochondrial integrity triggers numerous PRR immune surveillance programs, including the intracellular NOD, LRR, and pyrin domain-containing protein 3 (NLRP3) inflammasome and STING–interferon regulatory factor 3 (IRF3) pathways, as well as extracellular TLR9-initiated signaling. Perturbed mitochondrial fidelity can also increase reactive oxygen species (ROS) production, another strong driver of inflammasome activation and of cell injury and death [20.Sorbara M.T. Girardin S.E. Mitochondrial ROS fuel the inflammasome.Cell Res. 2011; 21: 558-560Crossref PubMed Scopus (193) Google Scholar,21.Liu T. et al.Imbalanced GSH/ROS and sequential cell death.J. Biochem. Mol. Toxicol. 2022; 36e22942Crossref Scopus (69) Google Scholar]. We propose that the crosstalk between liver parenchymal cells and resident and infiltrating immune cells can contribute to the progression of NAFLD to steatohepatitis (NASH). Moreover, given emerging evidence for mitochondrial dysfunction-initiated pathways, and considerable evidence supporting PRR signaling triggered by nutrient overload in the contribution to NASH progression, we discuss such evidence and highlight areas that require further study to demonstrate our hypothesis of an integrated module of nutrient overload, mitochondrial dysfunction, and PRR activation that drives liver inflammation. One hypothetical potential consequence of NAFLD-associated enlarged mitochondria, termed megamitochondria, is that these become resistant to engulfment by autophagosomes, and thereby preclude recycling/repair via selective mitophagy [22.Yamada T. et al.Prevention and regression of megamitochondria and steatosis by blocking mitochondrial fusion in the liver.iScience. 2022; 25103996Abstract Full Text Full Text PDF Scopus (10) Google Scholar]. This concept was explored in a methionine/choline-deficient dietary mouse model of NAFLD in which mitochondrial fusion was impaired by antisense knockdown of the fusion protein optic atrophy 1 (OPA1) [22.Yamada T. et al.Prevention and regression of megamitochondria and steatosis by blocking mitochondrial fusion in the liver.iScience. 2022; 25103996Abstract Full Text Full Text PDF Scopus (10) Google Scholar]. Preventing megamitochondria formation, as assessed by confocal immunofluorescence of liver sections, inhibited the development of steatosis and reversed established steatosis [22.Yamada T. et al.Prevention and regression of megamitochondria and steatosis by blocking mitochondrial fusion in the liver.iScience. 2022; 25103996Abstract Full Text Full Text PDF Scopus (10) Google Scholar], supporting the concept that disruption of mitochondrial quality control programs (mitochondrial fidelity) can contribute to disease progression. In parallel, attenuated mitochondrial fusion via OPA1 depletion blunted hepatic inflammation and fibrosis. Although mitochondrial integrity and specific inflammatory programs were not explored in this OPA1 knockdown model, the observed downregulation of the Il1b gene transcript encoding interleukin IL-1β following inhibition of mitochondrial fusion implicates the NLRP3 inflammasome in this mechanism, given that IL-1β is a canonical inflammasome-induced cytokine [22.Yamada T. et al.Prevention and regression of megamitochondria and steatosis by blocking mitochondrial fusion in the liver.iScience. 2022; 25103996Abstract Full Text Full Text PDF Scopus (10) Google Scholar]. Because autophagy processes (and their different specific iterations, e.g., mitophagy and lipophagy) are key mechanisms for removing intrinsic danger molecules across cells, it is not surprising that mitophagy impairment in the liver, in the context of NAFLD-associated mitochondrial infidelity, might contribute to inflammation in the transition to NASH (Box 2).Box 2Autophagic pathways linked to NAFLD and inflammationAutophagy (self-digestion) is a general homeostatic catabolic process that recycles intracellular content and organelles, and also removes intracellular danger components (pathogens, damaged organelles, unfolded proteins, etc.) by targeting them to lysosomes for destruction. Autophagosomes and lysosomes in the liver can play myriad roles, including quality control of mitochondria (mitophagy) and the catabolism of lipid stores (autophagy, selective lipophagy, and lysosomal lipolysis). These processes provide essential building blocks for liver function and more globally promote, in a context-specific manner, both pro- and anti-inflammatory effects [89.Das U.N. Essential fatty acids and their metabolites in the pathobiology of inflammation and its resolution.Biomolecules. 2021; 11: 1873Crossref PubMed Scopus (28) Google Scholar]. During liver inflammation, the accumulation of lipids, associated lipotoxicity, and compromised mitochondria across parenchymal and immune cells engage autophagy in mammalian cells to combat these disturbances and reinstate homeostasis [90.Czaja M.J. et al.Functions of autophagy in normal and diseased liver.Autophagy. 2013; 9: 1131-1158Crossref PubMed Scopus (366) Google Scholar]. Although the underlying molecular events are not well defined, this beneficial autophagic flux is gradually reduced during the progression to NASH, consequently leading to the accumulation of DAMPs, further mitochondrial disruption, and reduced cell viability [91.Lavallard V.J. Gual P. Autophagy and non-alcoholic fatty liver disease.Biomed. Res. Int. 2014; 2014120179Crossref PubMed Scopus (102) Google Scholar, 92.Ueno T. Komatsu M. Autophagy in the liver: functions in health and disease.Nat. Rev. Gastroenterol. Hepatol. 2017; 14: 170-184Crossref PubMed Scopus (337) Google Scholar, 93.Filali-Mouncef Y. et al.The menage a trois of autophagy, lipid droplets and liver disease.Autophagy. 2022; 18: 50-72Crossref PubMed Scopus (78) Google Scholar]. Similarly, DAMP-driven PRR activation can have a negative impact on autophagy, for example, in mouse oxidant-stressed cells, by reducing the expression of mitophagy activators [94.Sun Q. et al.Inflammasome and autophagy regulation – a two-way street.Mol. Med. 2017; 23: 188-195Crossref PubMed Scopus (132) Google Scholar]. Furthermore, NAFLD is generally associated with changes in autophagic (specifically lipophagic) machinery across the spectrum of parenchymal and immune cells in patients, as well as in several murine models of HFD-induced NAFLD [93.Filali-Mouncef Y. et al.The menage a trois of autophagy, lipid droplets and liver disease.Autophagy. 2022; 18: 50-72Crossref PubMed Scopus (78) Google Scholar]. Although targeting autophagy in NAFLD/NASH might be attractive (based on the central role of autophagy at the PRR–mitochondrion interface), this approach needs to be evaluated carefully because autophagy also serves important homeostatic and beneficial activities in the mammalian liver, such as the removal of compromised and non-functional mitochondria in the healthy liver and during distinct phases of NAFLD/NASH [95.Onishi M. et al.Molecular mechanisms and physiological functions of mitophagy.EMBO J. 2021; 40e104705Crossref PubMed Scopus (367) Google Scholar,96.Saha S. et al.Autophagy in health and disease: a comprehensive review.Biomed. Pharmacother. 2018; 104: 485-495Crossref PubMed Scopus (338) Google Scholar]. Autophagy (self-digestion) is a general homeostatic catabolic process that recycles intracellular content and organelles, and also removes intracellular danger components (pathogens, damaged organelles, unfolded proteins, etc.) by targeting them to lysosomes for destruction. Autophagosomes and lysosomes in the liver can play myriad roles, including quality control of mitochondria (mitophagy) and the catabolism of lipid stores (autophagy, selective lipophagy, and lysosomal lipolysis). These processes provide essential building blocks for liver function and more globally promote, in a context-specific manner, both pro- and anti-inflammatory effects [89.Das U.N. Essential fatty acids and their metabolites in the pathobiology of inflammation and its resolution.Biomolecules. 2021; 11: 1873Crossref PubMed Scopus (28) Google Scholar]. During liver inflammation, the accumulation of lipids, associated lipotoxicity, and compromised mitochondria across parenchymal and immune cells engage autophagy in mammalian cells to combat these disturbances and reinstate homeostasis [90.Czaja M.J. et al.Functions of autophagy in normal and diseased liver.Autophagy. 2013; 9: 1131-1158Crossref PubMed Scopus (366) Google Scholar]. Although the underlying molecular events are not well defined, this beneficial autophagic flux is gradually reduced during the progression to NASH, consequently leading to the accumulation of DAMPs, further mitochondrial disruption, and reduced cell viability [91.Lavallard V.J. Gual P. Autophagy and non-alcoholic fatty liver disease.Biomed. Res. Int. 2014; 2014120179Crossref PubMed Scopus (102) Google Scholar, 92.Ueno T. Komatsu M. Autophagy in the liver: functions in health and disease.Nat. Rev. Gastroenterol. Hepatol. 2017; 14: 170-184Crossref PubMed Scopus (337) Google Scholar, 93.Filali-Mouncef Y. et al.The menage a trois of autophagy, lipid droplets and liver disease.Autophagy. 2022; 18: 50-72Crossref PubMed Scopus (78) Google Scholar]. Similarly, DAMP-driven PRR activation can have a negative impact on autophagy, for example, in mouse oxidant-stressed cells, by reducing the expression of mitophagy activators [94.Sun Q. et al.Inflammasome and autophagy regulation – a two-way street.Mol. Med. 2017; 23: 188-195Crossref PubMed Scopus (132) Google Scholar]. Furthermore, NAFLD is generally associated with changes in autophagic (specifically lipophagic) machinery across the spectrum of parenchymal and immune cells in patients, as well as in several murine models of HFD-induced NAFLD [93.Filali-Mouncef Y. et al.The menage a trois of autophagy, lipid droplets and liver disease.Autophagy. 2022; 18: 50-72Crossref PubMed Scopus (78) Google Scholar]. Although targeting autophagy in NAFLD/NASH might be attractive (based on the central role of autophagy at the PRR–mitochondrion interface), this approach needs to be evaluated carefully because autophagy also serves important homeostatic and beneficial activities in the mammalian liver, such as the removal of compromised and non-functional mitochondria in the healthy liver and during distinct phases of NAFLD/NASH [95.Onishi M. et al.Molecular mechanisms and physiological functions of mitophagy.EMBO J. 2021; 40e104705Crossref PubMed Scopus (367) Google Scholar,96.Saha S. et al.Autophagy in health and disease: a comprehensive review.Biomed. Pharmacother. 2018; 104: 485-495Crossref PubMed Scopus (338) Google Scholar]. In primary hepatocytes and Kupffer